Atrial fibrillation is the most frequent sustained arrhythmia with a lifetime risk of 20-24% and is a significant cause of stroke, and other morbidity and mortality. There is strong evidence for heritability of AF, with a frequent intergenic chromosome 4 locus (4q25) being recently associated with AF in ambulatory populations. We have recently demonstrated that the same SNPs in the 4q25 locus, are also associated with new-onset postoperative AF in replicated cohorts of Caucasian patients after cardiac surgery. Thus, the genetic mechanisms of ambulatory AF and postoperative AF appear to be aligned, however, the mechanism(s) have not yet been identified. We will use the perioperative cardiac surgical environment, with its availability of atrial tissue and frequent AF phenotype to determine the biological mechanism(s) of the association between 4q25 genomic variation and AF. Our overall goal is to identify the genetic and molecular mechanisms whereby genetic variation in the 4q25 locus causes atrial fibrillation. In order to achieve this goal, we propose examining hypotheses for the association of 4q25 variants with AF: 1) We hypothesize that 4q25 variant(s) cause altered primary protein structure responsible for increased risk of AF. In order to identify 4q25 variants that lie within known and novel coding sequences we will measure novel atrial poly-A transcripts using Solexa sequencing of cDNA tags. We will then genotype all polymorphic coding variants in over 1900 previously collected cardiac surgical patients to identify all 4q25 coding variation associated with postoperative AF; 2) We hypothesize that 4q25 variants cause alternative splicing or differential expression of cis mRNA transcripts. We will identify 4q25 transcripts that are either differentially expressed or alternatively spliced, depending upon 4q25 AF-associated genotype, by mapping the 4q25 poly-A transcriptome of the human atrium using a structured cohort of 40 patients with and without postoperative AF, and Solexa mRNA sequencing; 3) We hypothesize 4q25 variants cause AF by regulation of mRNA translation by miRNA or other non-coding RNA. We will perform studies to identify non-coding RNA sources of regulation of 4q25 mRNA translation by resequencing all identified atrial poly-A and non poly-A RNA transcripts within 4q25 in the same structured cohort of 40 patients with and without postoperative AF. We will integrate the findings from the transcript and sequencing efforts to test the hypothesis. By performing these studies we will provide new insights into the molecular mechanisms that initiate AF. These insights may facilitate the development of novel therapeutic strategies to alleviate the burden of AF in the community, notably in the aging population.